German patent application No. 42 42 177 (P 7433) discloses a circuit arrangement of this type which is used to monitor valve coils of a controlled brake system and the associated actuating stages. The valve coils are connected to the positive pole of the vehicle battery by way of a joint main switch or a (semiconductor) relay, on the one hand, and grounded by way of each one final stage or an individual switch, on the other hand. To detect leakage currents, an auxiliary voltage source is connected, by way of a high-ohmic resistor, to the joint supply line which leads from the main switch to the valve coils. The potential on this supply line is measured and evaluated when the main switch is open. The potential on the supply line changes in the event of a shunt or leakage current from the supply line to the supply source or to ground. Shifts in potential which are due to leakage currents are detected and signalled by way of a voltage divider and a potential monitor.
Further, it has been disclosed in a circuit, which is similar to the above circuit in other respects, to connect the auxiliary voltage source that is used for testing purposes by way of a current source which can be comprised of two anti-parallel connected individual current sources. As long as the fault current is within predetermined limit values, the potential on the joint supply line remains approximately constant, while already an insignificant exceeding of these limit values will cause a remarkable (therefore easy to assess) potential variation on the supply line during the testing period (German patent application No. 195 26 435.5, not prior published).
U.S. Pat. No. 5,099,198 discloses a circuit for monitoring the final stages of a plurality of coils which permits detecting failure of the stages and the occurrence of leakage currents or shortcircuits. The final stages are actuated by test signals for error detection. The reactions caused thereby are identified by logical linking of the output signals and evaluated for error detection.
Further, EP-A-0 418 665 discloses a circuit for actuating and monitoring a plurality of coils and the associated final stages, for example, the valve coils of electrically controlled hydraulic valves of an anti-lock system, by way of a joint computer. Fault monitoring and fault determination is performed.
Assigned to each coil is a logic circuit including several detector circuits which respond to various functional faults of the coil circuits (such as shortcircuit, line interruption, permanent signal) . The associated logic circuits are actuated selectively by the joint control and test computer for monitoring the individual coils.
An object of the present invention is to develop a circuit arrangement of the above-mentioned type which permits detecting leakage currents or shunts of various types with a particularly high degree of reliability. Another important objective is a simple design and ease of analysis of the test results.
It has been found that this object is achieved by the circuit arrangement of the present invention which includes a current source as an auxiliary or test signal source which is connected to the supply voltage source, and is thereby activated, prior to each closing of the main switch, with the individual switches still open. Subsequently, the occurring voltage potential on the supply line is determined and evaluated for leakage current detection.
In the test period, i.e., after the activation of the testing current source, the potential on the supply line is increased until almost the total value of the supply voltage or battery voltage by the circuit arrangement according to the present invention. In contrast to known circuits with a low auxiliary voltage and/or high-ohmic resistors in the testing path, the increase of potential to the battery potential level also permits detecting shunts which occur or are caused only at a relatively high voltage level. Impurity spots and defects in the crystal lattice structure of semiconductors may cause shunts which become active only starting with a certain voltage potential. This behavior can be compared with the Zener effect where current flow occurs only above a defined voltage value, i.e., the Zener voltage.
In a preferred aspect of the present invention, the difference between the potential on the supply line and the potential of the supply voltage source is determined in the test period, i.e., after the activation of the testing current source, by way of a comparator. When the difference exceeds a defined, relatively low voltage threshold value, this indicates a shunt or a leakage current. This type of error detection is especially easy to achieve and it is reliable.
It is appropriate to evaluate the potential difference which is produced as a consequence of the activation of the testing current source only after a defined waiting time in order to also cover an induced current which flows via an inductive load, especially via one of the valve coils. This is because due to inductance the potential variation on the supply line will occur under certain conditions only after a defined delay time as a consequence of activation of the testing current source.